Currently, various display devices are used in a variety of applications. In commonly-used display devices, each pixel is composed of three subpixels for displaying three primaries of light, i.e., red, green and blue, whereby multicolor display is achieved.
However, conventional display devices have a problem in that they can only display colors in a narrow range (referred to as a “color gamut”). FIG. 17 shows a color gamut of a conventional display device which performs display by using three primaries. FIG. 17 is an xy chromaticity diagram in an XYZ color system, where a color gamut is shown by a triangle whose apices are at three points corresponding to the three primaries of red, green and blue. Also shown in the figure are plotted colors (represented by “×” symbols) of various objects existing in nature, as taught by Pointer (see Non-Patent Document 1). As can be seen from FIG. 17, there are some object colors which do not fall within the color gamut. Thus, display devices which perform display by using three primaries are unable to display some object colors.
Therefore, in order to broaden the color gamut of a display device, there has been proposed a technique which increases the number of primary colors to be used for displaying to four or more.
For example, as shown in FIG. 18, Patent Document 1 discloses a liquid crystal display device 800 each of whose pixels P is composed of six subpixels R, G, B, Y, C and M for displaying red, green, blue, yellow, cyan, and magenta. The color gamut of the liquid crystal display device 800 is shown in FIG. 19. As shown in FIG. 19, a color gamut which is represented as a hexagonal shape whose apices are at six points corresponding to the six primary colors essentially encompasses all object colors. Thus, the color gamut can be broadened by increasing the number of primary colors to be used for displaying. In the present specification, liquid crystal display devices which perform display by using three primary colors will be collectively referred to as “three-primary liquid crystal display devices”, and liquid crystal display devices which perform display by using four or more primary colors will be collectively referred to as “multiprimary liquid crystal display devices”.
However, sufficient display quality may not be achieved by merely increasing the number of primary colors. For example, in a liquid crystal display device 800 disclosed in Patent Document 1, the actually-displayed red colors will appear blackish red (i.e., dark red), which means that there actually exist some object colors that cannot be displayed. The reason why red appears blackish (darkened) in the liquid crystal display device 800 of Patent Document 1 is as follows.
When the number of primary colors to be used for displaying is increased, the number of subpixels per pixel increases, which inevitably reduces the area of each subpixel. This results in a lowered lightness (which corresponds to the Y value in the XYZ color system) of the color to be displayed by each subpixel. For example, if the number of primary colors used for displaying is increased from three to six, the area of each subpixel is reduced to about half, so that the lightness (Y value) of each subpixel is also reduced to about half.
“Lightness” is one of the three factors which define a color, besides “hue” and “chroma”. Therefore, even if the color gamut on the xy chromaticity diagram (i.e., the reproducible range of “hue” and “chroma”) may be broadened by increasing the number of primary colors as shown in FIG. 19, the lowered “lightness” prevents the actual color gamut (i.e., the color gamut which also takes “lightness” into account) from becoming sufficiently wide.
While subpixels for displaying green or blue can still sufficiently display various object colors under lowered lightness, the subpixels for displaying red will become unable to display some object colors under lowered lightness. Thus, if the lightness (Y value) becomes lower because of using an increased number of primary colors, the display quality of red is degraded such that red appears blackish red (i.e., dark red).
Techniques for solving this problem are proposed in Patent Documents 2 and 3. As is disclosed in Patent Documents 2 and 3, by providing two red subpixels in one pixel, the lightness (Y value) of red can be improved, thus making it possible to display bright red. In other words, it is possible to broaden the color gamut which takes lightness into account in addition to the hue and chroma represented on the xy chromaticity diagram. It is commonplace for the two red subpixels that are provided within the same pixel to be driven at the same gray scale level (same luminance) for circuit simplification.